Filter a Drop Down Menu Based on Related Items in a Many to Many Relationship

[davipilot]

I have two dataverse tables which we will call TableA and TableC.  They are related to each other through a Many to Many relationship.

I then have a Combo box to display the list of values from ColumnA in Table A.  A user will select a value from this list and save it which will now relate the selection from Table A to Table C.

What I would like to do and am unable to figure out, is how to filter out the values that are already related.

Example:

Initially the Combo box shows values of:

ValueA

ValueB

ValueC

ValueD

The user then relates ValueC to TableC and should only see the available values below.

ValueA

ValueB

ValueD